The idea that clusters form by gravitational clustering of field
galaxies led Zwicky
[513]
(and others) to suggest that cluster
galaxies are more massive than average, making their mutual
gravitational attraction stronger. The most massive
galaxies would cluster first, forming the cluster core, and other
galaxies would follow. Assuming proportionality between a galaxy
luminosity and its mass, Zwicky then thought that luminosity
segregation must exist in clusters. Between 1942 and 1951 he found
some evidence for it in Virgo
[515],
and in
Coma
[519].
At the same time he noted that also dwarf galaxies
are clustered(8),
an evidence later confirmed by
Reaves [366]
and Hodge [220].

Figure 10. The radial distribution of
bright and dwarf
galaxies in the Coma cluster. From Rood & Turnrose (1968).

In the sixties, Reaves
[368]
and Rood & Turnrose
[388]
showed that dwarf galaxies are less clustered than giant galaxies
- see Fig. 10.
Not much later, Rood
[381]
and Rood & Abell
[384] noted
that the bright peak in the luminosity function of Coma galaxies
(first described by Shapley
[416]
in 1934), is not present in the
outer regions of the cluster. This was interpreted as evidence for an
excess of bright galaxies in the cluster core, i.e. luminosity
segregation.

Oemler [331]
noted an increase of the mean radius of cluster
galaxies with galaxy magnitudes, another evidence for luminosity
segregation, which was not seen, however, in spiral-rich clusters.

Capelato et al.
[91]
examined in detail the luminosity
segregation in Coma, showing that it concerns the most luminous
galaxies in a range of about 2 magnitudes. They also enlightened the
role of the central cD in destroying the evidence of luminosity
segregation through cannibalism, as originally suggested by
Dressler [140].

Luminosity segregation also had opponents, like Noonan
[326],
Bahcall [40],
and Sarazin
[398],
who suggested the
evidence for luminosity segregation to be spurious, and mostly due to
poor background subtraction. Recent analyses
[69,
73], based
on cluster members only, show that luminosity segregation is indeed
limited to the very bright galaxies only,
MR < - 22.6.

8 Reaves noted that the main problems for
the identification of dwarf galaxies were their low surface brightness,
and the fact that these galaxies ``resemble water spots and
certain common emulsion defects''.Back.